Electrical fires in machinery frequently arise as a result of an electrical fault causing ignition and combustion of polymer materials, such as the insulating cladding of wires and cables. Such fires are a serious problem presenting a potential hazard to both workers and property, not only from the fire, but also from the evolution of toxic gases derived from the thermal degradation of the polymer materials. The problem may be compounded when access to the point of ignition is physically restricted, e.g., within machines,, circuitry etc., thereby limiting manual attempts to extinguish or even recognize the fire. Accordingly, there has been much interest in the development of polymer compositions having improved resistance to combustion.
It is known that blending certain compounds into a prepolymer composition can have a flame-retarding effect on the combustion of the final product. Halogen-containing flame-retardant additives are known and have been used to reduce the flammability of polymer materials. However, there is an increasing awareness of the problems associated with the use of halogenated flame-retardants. For example, halogenated flame-retardants are known to cause both high smoke generation and the emission of toxic gases which present a hazard to both workers and fire fighters alike, as well as corrosive gases which may damage adjacent circuitry. In addition, these gases may also have wider reaching deleterious environmental effects. Alternatives to halogenated flame-retardants can be broadly classed into either (a) phosphorus-based materials or (b) non-phosphorus-based materials.
Most non-phosphorus-based materials comprise inert filler materials, such as calcium carbonate, which have little anticombustion effect. Alumina trihydrate, magnesium hydroxide and calcium carbonate have been used as flame-retardant additives for polyolefin polymers. Unfortunately, such materials require high loading by weight of the polymer composition to achieve the desired level of flammability performance. This high loading is found to have a pronounced negative effect on the physical properties of the polymer, e.g., tensile strength and/or elongation. Additionally, materials such as alumina trihydrate have poor thermal stability.
The use of phosphorus-based flame-retardants is known, particularly the use of inorganic phosphates to generate intumescent formulations which form a protective foamed char when exposed to heat, thereby preventing further combustion. Such formulations have generally found applications in cellulosic type materials, resins and formulations containing low molecular weight polyols for paints, and other such coating materials. Various combinations of phosphorus-containing compounds are disclosed in, for example: Chemical Abstracts 92 111554 and 96 143939; U.S. Pat. Nos. 3485793,, 3541046, 3810862, 4001177, 4140660, 4182792, 4198493, 4341694, 4433115, 4491644, 4587362, 4642366 and 4742088; International Patent Nos. W085/05626 and W089/01011; British Patent Nos. 2142638 and 2181141; Australian Patent Application No. 77342/87, and European Patent Nos. 0026391, 0045835, 0115871, 0204027 and 0413613. However, these prior art flame-retardants are found to suffer from one or more of the following disadvantages:
(i) Water sensitivity.
(ii) Migration of the additive due to non-compatibility between the component compounds and the polymer matrix.
(iii) The deleterious action of the additive on the physical properties of the polymer.
(iv) The high specificity of the additive, for example, U.S. Pat. No. 4491644 discloses a flame-retardant additive comprising a salt formed from melamine and bis(pentaerythritol phosphate) phosphoric acid formulated for use principally in poly(propylene). Such materials are commercially available under the trade name CHARGARD. Likewise, European Patent No. 0115871 discloses flame-retardant additives comprising a nitrogen-containing oligomer and ammonium polyphosphate, which are Commercially available under the trade name SPINFLAM in grades specific to a particular polymer, e.g., polyethylene. As such, there is a continuing need for a high performance method of flame-retardation for use in polymers.